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Annotation of researchv10no/cmd/sml/src/coder/ieeereal.sml, revision 1.1

1.1     ! root        1: (* Copyright 1989 by AT&T Bell Laboratories *)
        !             2: (* ieeereal.sml
        !             3:  *
        !             4:  * J.H. Reppy
        !             5:  * Cornell University
        !             6:  * Ithaca, NY  14853
        !             7:  * [email protected]
        !             8:  *
        !             9:  * HISTORY:
        !            10:  *   03/15/89  created
        !            11:  *   11/20/89  changed argument sig of IEEEReal
        !            12:  *
        !            13:  * Support for IEEE floating-point constants (for M68881 and SPARC FPU).
        !            14:  *
        !            15:  * Double precision format (for normalized numbers):
        !            16:  *   Bias = 1023.
        !            17:  *   Exponent = 11 bits.
        !            18:  *   Range of exponent = [1..2046]
        !            19:  *   Mantissa = 52 (+1) bits.
        !            20:  *   Value = (-1)^s * 2^(e-1023) * 1.f
        !            21:  *
        !            22:  * Sub-normal numbers (biased exponent = 0)
        !            23:  *   Bias = 1022
        !            24:  *   Mantissa = 52 bits.
        !            25:  *   Value = (-1)^s * 2^-1022 * 0.f
        !            26:  *)
        !            27: 
        !            28: functor IEEEReal (val emitWord : int -> unit) : PRIMREAL =
        !            29: struct
        !            30: 
        !            31:     val significant = 53 (* 52 + redundant 1 bit *)
        !            32: 
        !            33:     fun outofrange s = ErrorMsg.complain("Real constant "^s^" out of range")
        !            34: 
        !            35:   (* Convert a portion of a boolean array to the appropriate integer. *)
        !            36:     exception Bits
        !            37:     fun bits(a, start, width) = let
        !            38:          fun b true = 1
        !            39:            | b false = 0
        !            40:          fun f 0 = b (a sub start)
        !            41:            | f n = b (a sub (start+n)) + 2 * f(n-1)
        !            42:          in
        !            43:            if (Array.length a < start+width) orelse (start < 0) orelse (width < 0)
        !            44:              then raise Bits
        !            45:              else f (width-1)
        !            46:          end
        !            47: 
        !            48:   (* Emit a real constant with the given sign, the mantissa frac and with the
        !            49:    * unbiased exponent exp.
        !            50:    *)
        !            51:     fun emitreal (sign, frac, exp) = let
        !            52:          val exponent = exp + 1022
        !            53:          fun emit () = let
        !            54:                val word0 = case frac sub 0 (* zero? *)
        !            55:                       of true => Bits.orb(Bits.lshift(sign,15),
        !            56:                            Bits.orb(Bits.lshift(exponent,4), bits(frac,1,4)))
        !            57:                        | false => 0
        !            58:                val word1 = bits(frac,5,16)
        !            59:                val word2 = bits(frac,21,16)
        !            60:                val word3 = bits(frac,37,16)
        !            61:                in
        !            62:                  emitWord word0;
        !            63:                  emitWord word1;
        !            64:                  emitWord word2;
        !            65:                  emitWord word3
        !            66:                end
        !            67:          in
        !            68:            if exponent < 1
        !            69:              then outofrange "" (** A sub-normal number **)
        !            70:              else if exponent > 2047
        !            71:                then outofrange "" (* A hack *)
        !            72:                else emit()
        !            73:          end
        !            74: 
        !            75: end (* functor IEEEReal *)

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